This invention relates to a method of injecting gel-forming chemicals into one or more relatively high permeability regions of an underground hydrocarbon bearing formation in order to reduce the permeability of these regions. This invention may therefore be used as a method of improving areal and vertical conformance and flow profiles at or away from a production and/or injection wellbore for the purpose of increasing hydrocarbon recovery.
Poor vertical conformance and excessive water production are problems encountered in secondary oil recovery processes in heterogeneous formations with high permeability contrast. This leads to poor recovery efficiencies by bypassing large concentrations of hydrocarbons, and the uneconomical production of high water-oil ratios fluids from producing wells. Most methods to improve conformance involve the injection of a chemical solution into the formation which forms a gel in high permeability regions and thereby lowers permeability.
A number of gel systems have been described over the years for improving conformance in subterranean hydrocarbon bearing formations. One approach commonly used for conformance improvement is injection of polyacrylamide based gels into the wellbore. Polyacrylamide gels are useful over a certain range of temperatures but at higher temperatures they undergo thermal hydrolysis which leads to Ca.sup.2+ and Mg.sup.2+ ion intolerance and/or gel degradation. Gel systems with increased stability, especially in the presence of high temperatures and hard water, were found to be desirable.
To address the need for conformance improvement of high temperature subterranean regions, other types of gel treatment methods were developed that make use of lignosulfonates and sulfonated lignins. These compounds have frequently been proposed to be used in permeability reducing processes due to their inexpensive cost and favorable chemical structure. The sulfonate groups of lignosulfonates do not complex readily with salts and therefore lignosulfonate gels are more resistant to precipitation due to hardness and high temperature than other types of gels. The disadvantage to this system is that the gelant solutions containing lignosulfonate are only set by high formation temperatures. U.S. Pat. No. 4,074,757 describes injecting lignosulfonate and water in the absence of other gelation promoters in order to achieve high temperature plugging. To achieve gelation at lower temperatures, lignosulfonates have been mixed with an activator comprised of dichromate (i.e. chromium (VI)) and an alkali metal or alkaline earth salt. U.S. Pat. No. 3,896,827 describes a lignosulfonate gel formed by injecting a lignosulfonate solution with a mixed activator comprising a dichromate and an alkali metal or alkaline earth salt. However, the use of dichromate is not environmentally preferred due to its high toxicity and carcinogenicity. Other variations of lignosulfonate based gelants have been described. U.S. Pat. Nos. 4,257,813 and 4,275,789 teach methods using silicate activated lignosulfonate gels. U.S. Pat. No. 4,428,429 discloses a process where a lignin solution gels in the formation. A profile control process is described in U.S. Pat. No. 4,110,231 where lignosulfonate and carboxymethyl cellulose comprise the treatment system.
Lignosulfonate-acrylic acid copolymers have been proposed for use as gels for conformance improvement. U.S. Pat. No. 4,721,161 discloses the use of lignosulfonate and acrylic acid which are reacted in situ using a catalyst and low pH to copolymerize the lignosulfonate and acrylic acid and form a gel to decrease permeability around a wellbore. However, due to the fast reaction rates involved in this method, the use of this system has been found to be undesirable in most instances. The fast reaction rates prevent the use of this method where deep penetration is required, such as injection well treatments for profile modification. In addition to limiting the amount of gel that can be used to treat the formation, the fast reaction rates of this gel system also necessitate the mixing of the gel components and catalyst within the wellbore. The use of gel treatment methods requiring in situ mixing of gel components have proven unsatisfactory in the art due to the difficulty in achieving complete mixing. Incomplete mixing in situ leads to uncontrolled and non-uniform gel placement in the underground formation.
The need therefore exists for a gel treatment system for conformance improvement that is stable over a wider range of temperatures and in the presence of hard water, and that can be delivered efficiently and controllably to the underground region in need of treatment.
These and other features of the present invention are more fully set forth in the following description of illustrative embodiments of the invention.